Reduced transaction I/O for log-structured storage systems

ABSTRACT

A distributed database system may implement reduced transaction I/O. In various embodiments, a plurality of log records may be stored in persistent storage for a plurality of pages of a log-structured storage system. A coalesce operation may be performed for at least some of the plurality of data pages for the log-structured storage system. The coalesce operation may, in some embodiments, coalesce two or more log records for one of the plurality of pages to generate an instance of the page and store the instance of the page to a new location in the persistent storage. Log records for one or more of the plurality of pages may be excluded from the coalesce operation. These excluded log records may not be dependent on a prior version of the page or portion of the page corresponding to the log record. Excluded log records may include one or more of redo records, undo records, or transaction table records.

BACKGROUND

Distribution of various components of a software stack can in some cases provide (or support) fault tolerance (e.g., through replication), higher durability, and less expensive solutions (e.g., through the use of many smaller, less-expensive components rather than fewer large, expensive components). However, databases have historically been among the components of the software stack that are least amenable to distribution. For example, it can difficult to distribute databases while still ensuring the so-called ACID properties (e.g., Atomicity, Consistency, Isolation, and Durability) that they are expected to provide.

While most existing relational databases are not distributed, some existing databases are “scaled out” (as opposed to being “scaled up” by merely employing a larger monolithic system) using one of two common models: a “shared nothing” model, and a “shared disk” model. In general, in a “shared nothing” model, received queries are decomposed into database shards (each of which includes a component of the query), these shards are sent to different compute nodes for query processing, and the results are collected and aggregated before they are returned. In general, in a “shared disk” model, every compute node in a cluster has access to the same underlying data. In systems that employ this model, great care must be taken to manage cache coherency. In both of these models, a large, monolithic database is replicated on multiple nodes (including all of the functionality of a stand-alone database instance), and “glue” logic is added to stitch them together. For example, in the “shared nothing” model, the glue logic may provide the functionality of a dispatcher that subdivides queries, sends them to multiple compute notes, and then combines the results. In a “shared disk” model, the glue logic may serve to fuse together the caches of multiple nodes (e.g., to manage coherency at the caching layer). These “shared nothing” and “shared disk” database systems can be costly to deploy, and complex to maintain, and may over-serve many database use cases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating various components of a database software stack, according to one embodiment.

FIG. 2 is a block diagram illustrating a service system architecture that may be configured to implement a web services-based database service, according to some embodiments.

FIG. 3 is a block diagram illustrating various components of a database system that includes a database engine and a separate distributed database storage service, according to one embodiment.

FIG. 4 is a block diagram illustrating a distributed database-optimized storage system, according to one embodiment.

FIG. 5 is a block diagram illustrating the use of a separate distributed database-optimized storage system in a database system, according to one embodiment.

FIG. 6 is a block diagram illustrating how data and metadata may be stored on a given node of a distributed database-optimized storage system, according to one embodiment.

FIG. 7 is a flow diagram illustrating a method for reduced transaction I/O in a log-structured system, according to some embodiments.

FIG. 8 is a flow diagram of a method to perform a coalesce operation for log records for a page of log-structured system, according to some embodiments.

FIG. 9A is a block diagram illustrating examples of reduced transaction I/O for log records for an undo log for a database, according to some embodiments.

FIG. 9B is a block diagram illustrating examples of reduced transaction I/O for transaction table records for a transaction table, according to some embodiments.

FIG. 10 is a flow diagram illustrating a method to perform reduced transaction I/O for an update to a database, according to some embodiments.

FIG. 11 illustrates an example computing system, according to some embodiments.

While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the embodiments are not limited to the embodiments or drawings described. It should be understood, that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The words “include,” “including,” and “includes” indicate open-ended relationships and therefore mean including, but not limited to. Similarly, the words “have,” “having,” and “has” also indicate open-ended relationships, and thus mean having, but not limited to. The terms “first,” “second,” “third,” and so forth as used herein are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.) unless such an ordering is otherwise explicitly indicated.

Various components may be described as “configured to” perform a task or tasks. In such contexts, “configured to” is a broad recitation generally meaning “having structure that” performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently performing that task (e.g., a computer system may be configured to perform operations even when the operations are not currently being performed). In some contexts, “configured to” may be a broad recitation of structure generally meaning “having circuitry that” performs the task or tasks during operation. As such, the component can be configured to perform the task even when the component is not currently on. In general, the circuitry that forms the structure corresponding to “configured to” may include hardware circuits.

Various components may be described as performing a task or tasks, for convenience in the description. Such descriptions should be interpreted as including the phrase “configured to.” Reciting a component that is configured to perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112, paragraph six, interpretation for that component.

“Based On.” As used herein, this term is used to describe one or more factors that affect a determination. This term does not foreclose additional factors that may affect a determination. That is, a determination may be solely based on those factors or based, at least in part, on those factors. Consider the phrase “determine A based on B.” While B may be a factor that affects the determination of A, such a phrase does not foreclose the determination of A from also being based on C. In other instances, A may be determined based solely on B.

The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.

DETAILED DESCRIPTION

Various embodiments of reduced transaction I/O for a log-structured storage system are disclosed. Log records for pages of a log-structured storage system may be stored in persistent storage. A coalesce operation may be performed for some of the pages of the log-structured storage. To perform a coalesce operation, two or more log records for a page may then be coalesced to generate an instance of the page. The instance of the page may then be stored to a new location in the persistent storage. Log records for one or more of the pages of the log-structured storage may be excluded from the coalesce operation. These log records may not be dependent on a prior version of the page or portion of the page corresponding to the log record. In various embodiments, log-structured storage systems may include various types of database or file systems. Excluded log records may include log records for the various types of database systems, such as redo records, undo records, and/or transaction table records, or log records for the various types of file systems, such as those that describe updates to files (e.g., file metadata).

The specification first describes an example web services-based database service configured to implement reduced transaction I/O for log-structured storage systems. Included in the description of the example web services-based database service are various aspects of the example web services-based database service, such as a database engine and a separate distributed database storage service. The specification then describes flowcharts of various embodiments of methods for reduced transaction I/O for log structured storage systems. Next, the specification describes an example system that may implement the disclosed techniques. Various examples are provided throughout the specification.

The systems described herein may, in some embodiments, implement a web service that enables clients (e.g., subscribers) to operate a data storage system in a cloud computing environment. In some embodiments, the data storage system may be an enterprise-class database system that is highly scalable and extensible. In some embodiments, queries may be directed to database storage that is distributed across multiple physical resources, and the database system may be scaled up or down on an as needed basis. The database system may work effectively with database schemas of various types and/or organizations, in different embodiments. In some embodiments, clients/subscribers may submit queries in a number of ways, e.g., interactively via an SQL interface to the database system. In other embodiments, external applications and programs may submit queries using Open Database Connectivity (ODBC) and/or Java Database Connectivity (JDBC) driver interfaces to the database system.

More specifically, the systems described herein may, in some embodiments, implement a service-oriented database architecture in which various functional components of a single database system are intrinsically distributed. For example, rather than lashing together multiple complete and monolithic database instances (each of which may include extraneous functionality, such as an application server, search functionality, or other functionality beyond that required to provide the core functions of a database), these systems may organize the basic operations of a database (e.g., query processing, transaction management, caching and storage) into tiers that may be individually and independently scalable. For example, in some embodiments, each database instance in the systems described herein may include a database tier (which may include a single database engine head node and a client-side storage system driver), and a separate, distributed storage system (which may include multiple storage nodes that collectively perform some of the operations traditionally performed in the database tier of existing systems).

As described in more detail herein, in some embodiments, some of the lowest level operations of a database, (e.g., backup, restore, snapshot, recovery, log record manipulation, and/or various space management operations) may be offloaded from the database engine to the storage layer and distributed across multiple nodes and storage devices. For example, in some embodiments, rather than the database engine applying changes to a database (or data pages thereof) and then sending the modified data pages to the storage layer, the application of changes to the stored database (and data pages thereof) may be the responsibility of the storage layer itself. In such embodiments, redo log records, rather than modified data pages, may be sent to the storage layer, after which redo processing (e.g., the application of the redo log records) may be performed somewhat lazily and in a distributed manner (e.g., by a background process). In some embodiments, crash recovery (e.g., the rebuilding of data pages from stored redo log records) may also be performed by the storage layer and may also be performed by a distributed (and, in some cases, lazy) background process.

In some embodiments, because only redo logs (and not modified data pages) are sent to the storage layer, there may be much less network traffic between the database tier and the storage layer than in existing database systems. In some embodiments, each redo log may be on the order of one-tenth the size of the corresponding data page for which it specifies a change. Note that requests sent from the database tier and the distributed storage system may be asynchronous and that multiple such requests may be in flight at a time.

In general, after being given a piece of data, a primary requirement of a database is that it can eventually give that piece of data back. To do this, the database may include several different components (or tiers), each of which performs a different function. For example, a traditional database may be thought of as having three tiers: a first tier for performing query parsing, optimization and execution; a second tier for providing transactionality, recovery, and durability; and a third tier that provides storage, either on locally attached disks or on network-attached storage. As noted above, previous attempts to scale a traditional database have typically involved replicating all three tiers of the database and distributing those replicated database instances across multiple machines.

In some embodiments, the systems described herein may partition functionality of a database system differently than in a traditional database, and may distribute only a subset of the functional components (rather than a complete database instance) across multiple machines in order to implement scaling. For example, in some embodiments, a client-facing tier may be configured to receive a request specifying what data is to be stored or retrieved, but not how to store or retrieve the data. This tier may perform request parsing and/or optimization (e.g., SQL parsing and optimization), while another tier may be responsible for query execution. In some embodiments, a third tier may be responsible for providing transactionality and consistency of results. For example, this tier may be configured to enforce some of the so-called ACID properties, in particular, the Atomicity of transactions that target the database, maintaining Consistency within the database, and ensuring Isolation between the transactions that target the database. In some embodiments, a fourth tier may then be responsible for providing Durability of the stored data in the presence of various sorts of faults. For example, this tier may be responsible for change logging, recovery from a database crash, managing access to the underlying storage volumes and/or space management in the underlying storage volumes.

Turning now to the figures, FIG. 1 is a block diagram illustrating various components of a database software stack, according to one embodiment. As illustrated in this example, a database instance may include multiple functional components (or layers), each of which provides a portion of the functionality of the database instance. In this example, database instance 100 includes a query parsing and query optimization layer (shown as 110), a query execution layer (shown as 120), a transactionality and consistency management layer (shown as 130), and a durability and space management layer (shown as 140). As noted above, in some existing database systems, scaling a database instance may involve duplicating the entire database instance one or more times (including all of the layers illustrated in FIG. 1), and then adding glue logic to stitch them together. In some embodiments, the systems described herein may instead offload the functionality of durability and space management layer 140 from the database tier to a separate storage layer, and may distribute that functionality across multiple storage nodes in the storage layer.

In some embodiments, the database systems described herein may retain much of the structure of the upper half of the database instance illustrated in FIG. 1, but may redistribute responsibility for at least portions of the backup, restore, snapshot, recovery, and/or various space management operations to the storage tier. Redistributing functionality in this manner and tightly coupling log processing between the database tier and the storage tier may improve performance, increase availability and reduce costs, when compared to previous approaches to providing a scalable database. For example, network and input/output bandwidth requirements may be reduced, since only redo log records (which are much smaller in size than the actual data pages) may be shipped across nodes or persisted within the latency path of write operations. In addition, the generation of data pages can be done independently in the background on each storage node (as foreground processing allows), without blocking incoming write operations. In some embodiments, the use of log-structured, non-overwrite storage may allow backup, restore, snapshots, point-in-time recovery, and volume growth operations to be performed more efficiently, e.g., by using metadata manipulation rather than movement or copying of a data page. In some embodiments, the storage layer may also assume the responsibility for the replication of data stored on behalf of clients (and/or metadata associated with that data, such as redo log records) across multiple storage nodes. For example, data (and/or metadata) may be replicated locally (e.g., within a single “availability zone” in which a collection of storage nodes executes on its own physically distinct, independent infrastructure) and/or across availability zones in a single region or in different regions.

In various embodiments, the database systems described herein may support a standard or custom application programming interface (API) for a variety of database operations. For example, the API may support operations for creating a database, creating a table, altering a table, creating a user, dropping a user, inserting one or more rows in a table, copying values, selecting data from within a table (e.g., querying a table), canceling or aborting a query, creating a snapshot, and/or other operations.

In some embodiments, the database tier of a database instance may include a database engine head node server that receives read and/or write requests from various client programs (e.g., applications) and/or subscribers (users), then parses them and develops an execution plan to carry out the associated database operation(s). For example, the database engine head node may develop the series of steps necessary to obtain results for complex queries and joins. In some embodiments, the database engine head node may manage communications between the database tier of the database system and clients/subscribers, as well as communications between the database tier and a separate distributed database-optimized storage system.

In some embodiments, the database engine head node may be responsible for receiving SQL requests from end clients through a JDBC or ODBC interface and for performing SQL processing and transaction management (which may include locking) locally. However, rather than generating data pages locally, the database engine head node (or various components thereof) may generate redo log records and may ship them to the appropriate nodes of a separate distributed storage system. In some embodiments, a client-side driver for the distributed storage system may be hosted on the database engine head node and may be responsible for routing redo log records to the storage system node (or nodes) that store the segments (or data pages thereof) to which those redo log records are directed. For example, in some embodiments, each segment may be mirrored (or otherwise made durable) on multiple storage system nodes that form a protection group. In such embodiments, the client-side driver may keep track of the nodes on which each segment is stored and may route redo logs to all of the nodes on which a segment is stored (e.g., asynchronously and in parallel, at substantially the same time), when a client request is received. As soon as the client-side driver receives an acknowledgement back from a write quorum of the storage nodes in the protection group (which may indicate that the redo log record has been written to the storage node), it may send an acknowledgement of the requested change to the database tier (e.g., to the database engine head node). For example, in embodiments in which data is made durable through the use of protection groups, the database engine head node may not be able to commit a transaction until and unless the client-side driver receives a reply from enough storage node instances to constitute a write quorum. Similarly, for a read request directed to a particular segment, the client-side driver may route the read request to all of the nodes on which the segment is stored (e.g., asynchronously and in parallel, at substantially the same time). As soon as the client-side driver receives the requested data from a read quorum of the storage nodes in the protection group, it may return the requested data to the database tier (e.g., to the database engine head node).

In some embodiments, the database tier (or more specifically, the database engine head node) may include a cache in which recently accessed data pages are held temporarily. In such embodiments, if a write request is received that targets a data page held in such a cache, in addition to shipping a corresponding redo log record to the storage layer, the database engine may apply the change to the copy of the data page held in its cache. However, unlike in other database systems, a data page held in this cache may not ever be flushed to the storage layer, and it may be discarded at any time (e.g., at any time after the redo log record for a write request that was most recently applied to the cached copy has been sent to the storage layer and acknowledged). The cache may implement any of various locking mechanisms to control access to the cache by at most one writer (or multiple readers) at a time, in different embodiments. Note, however, that in embodiments that include such a cache, the cache may not be distributed across multiple nodes, but may exist only on the database engine head node for a given database instance. Therefore, there may be no cache coherency or consistency issues to manage.

In some embodiments, the database tier may support the use of synchronous or asynchronous read replicas in the system, e.g., read-only copies of data on different nodes of the database tier to which read requests can be routed. In such embodiments, if the database engine head node for a given database receives a read request directed to a particular data page, it may route the request to any one (or a particular one) of these read-only copies. In some embodiments, the client-side driver in the database engine head node may be configured to notify these other nodes about updates and/or invalidations to cached data pages (e.g., in order to prompt them to invalidate their caches, after which they may request updated copies of updated data pages from the storage layer).

In some embodiments, the client-side driver running on the database engine head node may expose a private interface to the storage tier. In some embodiments, it may also expose a traditional iSCSI interface to one or more other components (e.g., other database engines or virtual computing services components). In some embodiments, storage for a database instance in the storage tier may be modeled as a single volume that can grow in size without limits, and that can have an unlimited number of IOPS associated with it. When a volume is created, it may be created with a specific size, with a specific availability/durability characteristic (e.g., specifying how it is replicated), and/or with an IOPS rate associated with it (e.g., both peak and sustained). For example, in some embodiments, a variety of different durability models may be supported, and users/subscribers may be able to specify, for their database, a number of replication copies, zones, or regions and/or whether replication is synchronous or asynchronous based upon their durability, performance and cost objectives.

In some embodiments, the client side driver may maintain metadata about the volume and may directly send asynchronous requests to each of the storage nodes necessary to fulfill read requests and write requests without requiring additional hops between storage nodes. For example, in some embodiments, in response to a request to make a change to a database, the client-side driver may be configured to determine the one or more nodes that are implementing the storage for the targeted data page, and to route the redo log record(s) specifying that change to those storage nodes. The storage nodes may then be responsible for applying the change specified in the redo log record to the targeted data page at some point in the future. As writes are acknowledged back to the client-side driver, the client-side driver may advance the point at which the volume is durable and may acknowledge commits back to the database tier. As previously noted, in some embodiments, the client-side driver may not ever send data pages to the storage node servers. This may not only reduce network traffic, but may also remove the need for the checkpoint or background writer threads that constrain foreground-processing throughput in previous database systems.

In some embodiments, many read requests may be served by the database engine head node cache. However, write requests may require durability, since large-scale failure events may be too common to allow only in-memory replication. Therefore, the systems described herein may be configured to minimize the cost of the redo log record write operations that are in the foreground latency path by implementing data storage in the storage tier as two regions: a small append-only log-structured region into which redo log records are written when they are received from the database tier, and a larger region in which log records are coalesced together to create new versions of data pages in the background. In some embodiments, an in-memory structure may be maintained for each data page that points to the last redo log record for that page, backward chaining log records until an instantiated data block is referenced. This approach may provide good performance for mixed read-write workloads, including in applications in which reads are largely cached.

In some embodiments, because accesses to the log-structured data storage for the redo log records may consist of a series of sequential input/output operations (rather than random input/output operations), the changes being made may be tightly packed together. It should also be noted that, in contrast to existing systems in which each change to a data page results in two input/output operations to persistent data storage (one for the redo log and one for the modified data page itself), in some embodiments, the systems described herein may avoid this “write amplification” by coalescing data pages at the storage nodes of the distributed storage system based on receipt of the redo log records.

As previously noted, in some embodiments, the storage tier of the database system may be responsible for taking database snapshots. However, because the storage tier implements log-structured storage, taking a snapshot of a data page (e.g., a data block) may include recording a timestamp associated with the redo log record that was most recently applied to the data page/block (or a timestamp associated with the most recent operation to coalesce multiple redo log records to create a new version of the data page/block), and preventing garbage collection of the previous version of the page/block and any subsequent log entries up to the recorded point in time. In such embodiments, taking a database snapshot may not require reading, copying, or writing the data block, as would be required when employing an off-volume backup strategy. In some embodiments, the space requirements for snapshots may be minimal, since only modified data would require additional space, although user/subscribers may be able to choose how much additional space they want to keep for on-volume snapshots in addition to the active data set. In different embodiments, snapshots may be discrete (e.g., each snapshot may provide access to all of the data in a data page as of a specific point in time) or continuous (e.g., each snapshot may provide access to all versions of the data that existing in a data page between two points in time). In some embodiments, reverting to a prior snapshot may include recording a log record to indicate that all redo log records and data pages since that snapshot are invalid and garbage collectable, and discarding all database cache entries after the snapshot point. In such embodiments, no roll-forward may be required since the storage system will, on a block-by-block basis, apply redo log records to data blocks as requested and in the background across all nodes, just as it does in normal forward read/write processing. Crash recovery may thereby be made parallel and distributed across nodes.

One embodiment of a service system architecture that may be configured to implement a web services-based database service is illustrated in FIG. 2. In the illustrated embodiment, a number of clients (shown as database clients 250 a-250 n) may be configured to interact with a web services platform 200 via a network 260. Web services platform 200 may be configured to interface with one or more instances of a database service 210, a distributed database-optimized storage service 220 and/or one or more other virtual computing services 230. It is noted that where one or more instances of a given component may exist, reference to that component herein may be made in either the singular or the plural. However, usage of either form is not intended to preclude the other.

In various embodiments, the components illustrated in FIG. 2 may be implemented directly within computer hardware, as instructions directly or indirectly executable by computer hardware (e.g., a microprocessor or computer system), or using a combination of these techniques. For example, the components of FIG. 2 may be implemented by a system that includes a number of computing nodes (or simply, nodes), each of which may be similar to the computer system embodiment illustrated in FIG. 11 and described below. In various embodiments, the functionality of a given service system component (e.g., a component of the database service or a component of the storage service) may be implemented by a particular node or may be distributed across several nodes. In some embodiments, a given node may implement the functionality of more than one service system component (e.g., more than one database service system component).

Generally speaking, clients 250 may encompass any type of client configurable to submit web services requests to web services platform 200 via network 260, including requests for database services (e.g., a request to generate a snapshot, etc.). For example, a given client 250 may include a suitable version of a web browser, or may include a plug-in module or other type of code module configured to execute as an extension to or within an execution environment provided by a web browser. Alternatively, a client 250 (e.g., a database service client) may encompass an application such as a database application (or user interface thereof), a media application, an office application or any other application that may make use of persistent storage resources to store and/or access one or more databases. In some embodiments, such an application may include sufficient protocol support (e.g., for a suitable version of Hypertext Transfer Protocol (HTTP)) for generating and processing web services requests without necessarily implementing full browser support for all types of web-based data. That is, client 250 may be an application configured to interact directly with web services platform 200. In some embodiments, client 250 may be configured to generate web services requests according to a Representational State Transfer (REST)-style web services architecture, a document- or message-based web services architecture, or another suitable web services architecture.

In some embodiments, a client 250 (e.g., a database service client) may be configured to provide access to web services-based storage of databases to other applications in a manner that is transparent to those applications. For example, client 250 may be configured to integrate with an operating system or file system to provide storage in accordance with a suitable variant of the storage models described herein. However, the operating system or file system may present a different storage interface to applications, such as a conventional file system hierarchy of files, directories and/or folders. In such an embodiment, applications may not need to be modified to make use of the storage system service model of FIG. 1. Instead, the details of interfacing to Web services platform 200 may be coordinated by client 250 and the operating system or file system on behalf of applications executing within the operating system environment.

Clients 250 may convey web services requests (e.g., a snapshot request, parameters of a snapshot request, read request, restore a snapshot, etc.) to and receive responses from web services platform 200 via network 260. In various embodiments, network 260 may encompass any suitable combination of networking hardware and protocols necessary to establish web-based communications between clients 250 and platform 200. For example, network 260 may generally encompass the various telecommunications networks and service providers that collectively implement the Internet. Network 260 may also include private networks such as local area networks (LANs) or wide area networks (WANs) as well as public or private wireless networks. For example, both a given client 250 and web services platform 200 may be respectively provisioned within enterprises having their own internal networks. In such an embodiment, network 260 may include the hardware (e.g., modems, routers, switches, load balancers, proxy servers, etc.) and software (e.g., protocol stacks, accounting software, firewall/security software, etc.) necessary to establish a networking link between given client 250 and the Internet as well as between the Internet and web services platform 200. It is noted that in some embodiments, clients 250 may communicate with web services platform 200 using a private network rather than the public Internet. For example, clients 250 may be provisioned within the same enterprise as a database service system (e.g., a system that implements database service 210 and/or distributed database-optimized storage service 220). In such a case, clients 250 may communicate with platform 200 entirely through a private network 260 (e.g., a LAN or WAN that may use Internet-based communication protocols but which is not publicly accessible).

Generally speaking, web services platform 200 may be configured to implement one or more service endpoints configured to receive and process web services requests, such as requests to access data pages (or records thereof). For example, web services platform 200 may include hardware and/or software configured to implement a particular endpoint, such that an HTTP-based web services request directed to that endpoint is properly received and processed. In one embodiment, web services platform 200 may be implemented as a server system configured to receive web services requests from clients 250 and to forward them to components of a system that implements database service 210, distributed database-optimized storage service 220 and/or another virtual computing service 230 for processing. In other embodiments, web services platform 200 may be configured as a number of distinct systems (e.g., in a cluster topology) implementing load balancing and other request management features configured to dynamically manage large-scale web services request processing loads. In various embodiments, web services platform 200 may be configured to support REST-style or document-based (e.g., SOAP-based) types of web services requests.

In addition to functioning as an addressable endpoint for clients' web services requests, in some embodiments, web services platform 200 may implement various client management features. For example, platform 200 may coordinate the metering and accounting of client usage of web services, including storage resources, such as by tracking the identities of requesting clients 250, the number and/or frequency of client requests, the size of data tables (or records thereof) stored or retrieved on behalf of clients 250, overall storage bandwidth used by clients 250, class of storage requested by clients 250, or any other measurable client usage parameter. Platform 200 may also implement financial accounting and billing systems, or may maintain a database of usage data that may be queried and processed by external systems for reporting and billing of client usage activity. In certain embodiments, platform 200 may be configured to collect, monitor and/or aggregate a variety of storage service system operational metrics, such as metrics reflecting the rates and types of requests received from clients 250, bandwidth utilized by such requests, system processing latency for such requests, system component utilization (e.g., network bandwidth and/or storage utilization within the storage service system), rates and types of errors resulting from requests, characteristics of stored and requested data pages or records thereof (e.g., size, data type, etc.), or any other suitable metrics. In some embodiments such metrics may be used by system administrators to tune and maintain system components, while in other embodiments such metrics (or relevant portions of such metrics) may be exposed to clients 250 to enable such clients to monitor their usage of database service 210, distributed database-optimized storage service 220 and/or another virtual computing service 230 (or the underlying systems that implement those services).

In some embodiments, platform 200 may also implement user authentication and access control procedures. For example, for a given web services request to access a particular database, platform 200 may be configured to ascertain whether the client 250 associated with the request is authorized to access the particular database. Platform 200 may determine such authorization by, for example, evaluating an identity, password or other credential against credentials associated with the particular database, or evaluating the requested access to the particular database against an access control list for the particular database. For example, if a client 250 does not have sufficient credentials to access the particular database, platform 200 may reject the corresponding web services request, for example by returning a response to the requesting client 250 indicating an error condition. Various access control policies may be stored as records or lists of access control information by database service 210, distributed database-optimized storage service 220 and/or other virtual computing services 230.

It is noted that while web services platform 200 may represent the primary interface through which clients 250 may access the features of a database system that implements database service 210, it need not represent the sole interface to such features. For example, an alternate API that may be distinct from a web services interface may be used to allow clients internal to the enterprise providing the database system to bypass web services platform 200. Note that in many of the examples described herein, distributed database-optimized storage service 220 may be internal to a computing system or an enterprise system that provides database services to clients 250, and may not be exposed to external clients (e.g., users or client applications). In such embodiments, the internal “client” (e.g., database service 210) may access distributed database-optimized storage service 220 over a local or private network, shown as the solid line between distributed database-optimized storage service 220 and database service 210 (e.g., through an API directly between the systems that implement these services). In such embodiments, the use of distributed database-optimized storage service 220 in storing databases on behalf of clients 250 may be transparent to those clients. In other embodiments, distributed database-optimized storage service 220 may be exposed to clients 250 through web services platform 200 to provide storage of databases or other information for applications other than those that rely on database service 210 for database management. This is illustrated in FIG. 2 by the dashed line between web services platform 200 and distributed database-optimized storage service 220. In such embodiments, clients of the distributed database-optimized storage service 220 may access distributed database-optimized storage service 220 via network 260 (e.g., over the Internet). In some embodiments, a virtual computing service 230 may be configured to receive storage services from distributed database-optimized storage service 220 (e.g., through an API directly between the virtual computing service 230 and distributed database-optimized storage service 220) to store objects used in performing computing services 230 on behalf of a client 250. This is illustrated in FIG. 2 by the dashed line between virtual computing service 230 and distributed database-optimized storage service 220. In some cases, the accounting and/or credentialing services of platform 200 may be unnecessary for internal clients such as administrative clients or between service components within the same enterprise.

Note that in various embodiments, different storage policies may be implemented by database service 210 and/or distributed database-optimized storage service 220. Examples of such storage policies may include a durability policy (e.g., a policy indicating the number of instances of a database (or data page thereof) that will be stored and the number of different nodes on which they will be stored) and/or a load balancing policy (which may distribute databases, or data pages thereof, across different nodes, volumes and/or disks in an attempt to equalize request traffic). In addition, different storage policies may be applied to different types of stored items by various one of the services. For example, in some embodiments, distributed database-optimized storage service 220 may implement a higher durability for redo log records than for data pages.

FIG. 3 is a block diagram illustrating various components of a database system that includes a database engine and a separate distributed database storage service, according to one embodiment. In this example, database system 300 includes a respective database engine head node 320 for each of several databases and a distributed database-optimized storage service 310 (which may or may not be visible to the clients of the database system, shown as database clients 350 a-350 n). As illustrated in this example, one or more of database clients 350 a-350 n may access a database head node 320 (e.g., head node 320 a, head node 320 b, or head node 320 c, each of which is a component of a respective database instance) via network 360 (e.g., these components may be network-addressable and accessible to the database clients 350 a-350 n). However, distributed database-optimized storage service 310, which may be employed by the database system to store data pages of one or more databases (and redo log records and/or other metadata associated therewith) on behalf of database clients 350 a-350 n, and to perform other functions of the database system as described herein, may or may not be network-addressable and accessible to the storage clients 350 a-350 n, in different embodiments. For example, in some embodiments, distributed database-optimized storage service 310 may perform various storage, access, change logging, recovery, log record manipulation, and/or space management operations in a manner that is invisible to storage clients 350 a-350 n.

As previously noted, each database instance may include a single database engine head node 320 that receives requests (e.g., a snapshot request, etc.) from various client programs (e.g., applications) and/or subscribers (users), then parses them, optimizes them, and develops an execution plan to carry out the associated database operation(s). In the example illustrated in FIG. 3, a query parsing, optimization, and execution component 305 of database engine head node 320 a may perform these functions for queries that are received from database client 350 a and that target the database instance of which database engine head node 320 a is a component. In some embodiments, query parsing, optimization, and execution component 305 may return query responses to database client 350 a, which may include write acknowledgements, requested data pages (or portions thereof), error messages, and or other responses, as appropriate. As illustrated in this example, database engine head node 320 a may also include a client-side storage service driver 325, which may route read requests and/or redo log records to various storage nodes within distributed database-optimized storage service 310, receive write acknowledgements from distributed database-optimized storage service 310, receive requested data pages from distributed database-optimized storage service 310, and/or return data pages, error messages, or other responses to query parsing, optimization, and execution component 305 (which may, in turn, return them to database client 350 a).

In this example, database engine head node 320 a includes a data page cache 335, in which data pages that were recently accessed may be temporarily held. As illustrated in FIG. 3, database engine head node 320 a may also include a transaction and consistency management component 330, which may be responsible for providing transactionality and consistency in the database instance of which database engine head node 320 a is a component. For example, this component may be responsible for ensuring the Atomicity, Consistency, and Isolation properties of the database instance and the transactions that are directed that the database instance. As illustrated in FIG. 3, database engine head node 320 a may also include a transaction log 340 and an undo log 345, which may be employed by transaction and consistency management component 330 to track the status of various transactions and roll back any locally cached results of transactions that do not commit.

Note that each of the other database engine head nodes 320 illustrated in FIG. 3 (e.g., 320 b and 320 c) may include similar components and may perform similar functions for queries received by one or more of database clients 350 a-350 n and directed to the respective database instances of which it is a component.

In some embodiments, the distributed database-optimized storage systems described herein may organize data in various logical volumes, segments, and pages for storage on one or more storage nodes. For example, in some embodiments, each database is represented by a logical volume, and each logical volume is segmented over a collection of storage nodes. Each segment, which lives on a particular one of the storage nodes, contains a set of contiguous block addresses. In some embodiments, each data page is stored in a segment, such that each segment stores a collection of one or more data pages and a change log (also referred to as a redo log) (e.g., a log of redo log records) for each data page that it stores. As described in detail herein, the storage nodes may be configured to receive log records (which may also be referred to herein as ULRs) and to coalesce them to create new versions of the corresponding data pages and/or additional or replacement log records (e.g., lazily and/or in response to a request for a data page or a database crash). In some embodiments, data pages and/or change logs may be mirrored across multiple storage nodes, according to a variable configuration (which may be specified by the client on whose behalf the databases are being maintained in the database system). For example, in different embodiments, one, two, or three copies of the data or change logs may be stored in each of one, two, or three different availability zones or regions, according to a default configuration, an application-specific durability preference, or a client-specified durability preference.

As used herein, the following terms may be used to describe the organization of data by a distributed database-optimized storage system, according to various embodiments.

Volume: A volume is a logical concept representing a highly durable unit of storage that a user/client/application of the storage system understands. More specifically, a volume is a distributed store that appears to the user/client/application as a single consistent ordered log of write operations to various user pages of a database. Each write operation may be encoded in a User Log Record (ULR), which represents a logical, ordered mutation to the contents of a single user page within the volume. As noted above, a ULR may also be referred to herein as a redo log record. Each ULR may include a unique identifier (e.g., a Logical Sequence Number (LSN)). Each ULR may be persisted to one or more synchronous segments in the distributed store that form a Protection Group (PG), to provide high durability and availability for the ULR. A volume may provide an LSN-type read/write interface for a variable-size contiguous range of bytes.

In some embodiments, a volume may consist of multiple extents, each made durable through a protection group. In such embodiments, a volume may represent a unit of storage composed of a mutable contiguous sequence of Volume Extents. Reads and writes that are directed to a volume may be mapped into corresponding reads and writes to the constituent volume extents. In some embodiments, the size of a volume may be changed by adding or removing volume extents from the end of the volume.

Segment: A segment is a limited-durability unit of storage assigned to a single storage node. More specifically, a segment provides limited best-effort durability (e.g., a persistent, but non-redundant single point of failure that is a storage node) for a specific fixed-size byte range of data. This data may in some cases be a mirror of user-addressable data, or it may be other data, such as volume metadata or erasure coded bits, in various embodiments. A given segment may live on exactly one storage node. Within a storage node, multiple segments may live on each SSD, and each segment may be restricted to one SSD (e.g., a segment may not span across multiple SSDs). In some embodiments, a segment may not be required to occupy a contiguous region on an SSD; rather there may be an allocation map in each SSD describing the areas that are owned by each of the segments. As noted above, a protection group may consist of multiple segments spread across multiple storage nodes. In some embodiments, a segment may provide an LSN-type read/write interface for a fixed-size contiguous range of bytes (where the size is defined at creation). In some embodiments, each segment may be identified by a Segment UUID (e.g., a universally unique identifier of the segment).

Storage page: A storage page is a block of memory, generally of fixed size. In some embodiments, each page is a block of memory (e.g., of virtual memory, disk, or other physical memory) of a size defined by the operating system, and may also be referred to herein by the term “data block”. More specifically, a storage page may be a set of contiguous sectors. It may serve as the unit of allocation in SSDs, as well as the unit in log pages for which there is a header and metadata. In some embodiments, and in the context of the database systems described herein, the term “page” or “storage page” may refer to a similar block of a size defined by the database configuration, which may typically a multiple of 2, such as 4096, 8192, 16384, or 32768 bytes.

Log page: A log page is a type of storage page that is used to store log records (e.g., redo log records or undo log records). In some embodiments, log pages may be identical in size to storage pages. Each log page may include a header containing metadata about that log page, e.g., metadata identifying the segment to which it belongs. Note that a log page is a unit of organization and may not necessarily be the unit of data included in write operations. For example, in some embodiments, during normal forward processing, write operations may write to the tail of the log one sector at a time.

Log Records: Log records (e.g., the individual elements of a log page) may be of several different classes. For example, User Log Records (ULRs), which are created and understood by users/clients/applications of the storage system, may be used to indicate changes to user data in a volume. Control Log Records (CLRs), which are generated by the storage system, may contain control information used to keep track of metadata such as the current unconditional volume durable LSN (VDL). Null Log Records (NLRs) may in some embodiments be used as padding to fill in unused space in a log sector or log page. In some embodiments, there may be various types of log records within each of these classes, and the type of a log record may correspond to a function that needs to be invoked to interpret the log record. For example, one type may represent all the data of a user page in compressed format using a specific compression format; a second type may represent new values for a byte range within a user page; a third type may represent an increment operation to a sequence of bytes interpreted as an integer; and a fourth type may represent copying one byte range to another location within the page. In some embodiments, log record types may be identified by GUIDs (rather than by integers or enums), which may simplify versioning and development, especially for ULRs.

Payload: The payload of a log record is the data or parameter values that are specific to the log record or to log records of a particular type. For example, in some embodiments, there may be a set of parameters or attributes that most (or all) log records include, and that the storage system itself understands. These attributes may be part of a common log record header/structure, which may be relatively small compared to the sector size. In addition, most log records may include additional parameters or data specific to that log record type, and this additional information may be considered the payload of that log record. In some embodiments, if the payload for a particular ULR is larger than the user page size, it may be replaced by an absolute ULR (an AULR) whose payload includes all the data for the user page. This may enable the storage system to enforce an upper limit on the size of the payload for ULRs that is equal to the size of user pages.

Note that when storing log records in the segment log, the payload may be stored along with the log header, in some embodiments. In other embodiments, the payload may be stored in a separate location, and pointers to the location at which that payload is stored may be stored with the log header. In still other embodiments, a portion of the payload may be stored in the header, and the remainder of the payload may be stored in a separate location. If the entire payload is stored with the log header, this may be referred to as in-band storage; otherwise the storage may be referred to as being out-of-band. In some embodiments, the payloads of most large AULRs may be stored out-of-band in the cold zone of log (which is described below).

User pages: User pages are the byte ranges (of a fixed size) and alignments thereof for a particular volume that are visible to users/clients of the storage system. User pages are a logical concept, and the bytes in particular user pages may or not be stored in any storage page as-is. The size of the user pages for a particular volume may be independent of the storage page size for that volume. In some embodiments, the user page size may be configurable per volume, and different segments on a storage node may have different user page sizes. In some embodiments, user page sizes may be constrained to be a multiple of the sector size (e.g., 4 KB), and may have an upper limit (e.g., 64 KB). The storage page size, on the other hand, may be fixed for an entire storage node and may not change unless there is a change to the underlying hardware.

Data page: A data page is a type of storage page that is used to store user page data in compressed form. In some embodiments every piece of data stored in a data page is associated with a log record, and each log record may include a pointer to a sector within a data page (also referred to as a data sector). In some embodiments, data pages may not include any embedded metadata other than that provided by each sector. There may be no relationship between the sectors in a data page. Instead, the organization into pages may exist only as an expression of the granularity of the allocation of data to a segment.

Storage node: A storage node is a single virtual machine that on which storage node server code is deployed. Each storage node may contain multiple locally attached SSDs, and may provide a network API for access to one or more segments. In some embodiments, various nodes may be on an active list or on a degraded list (e.g., if they are slow to respond or are otherwise impaired, but are not completely unusable). In some embodiments, the client-side driver may assist in (or be responsible for) classifying nodes as active or degraded, for determining if and when they should be replaced, and/or for determining when and how to redistribute data among various nodes, based on observed performance.

SSD: As referred to herein, the term “SSD” may refer to a local block storage volume as seen by the storage node, regardless of the type of storage employed by that storage volume, e.g., disk, a solid-state drive, a battery-backed RAM, a non-volatile RAM device (e.g., one or more NV-DIMMs) or another type of persistent storage device. An SSD is not necessarily mapped directly to hardware. For example, a single solid-state storage device might be broken up into multiple local volumes where each volume is split into and striped across multiple segments, and/or a single drive may be broken up into multiple volumes simply for ease of management, in different embodiments. In some embodiments, each SSD may store an allocation map at a single fixed location. This map may indicate which storage pages that are owned by particular segments, and which of these pages are log pages (as opposed to data pages). In some embodiments, storage pages may be pre-allocated to each segment so that forward processing may not need to wait for allocation. Any changes to the allocation map may need to be made durable before newly allocated storage pages are used by the segments.

One embodiment of a distributed database-optimized storage system is illustrated by the block diagram in FIG. 4. In this example, a database system 400 includes a distributed database-optimized storage system 410, which communicates with a database engine head node 420 over interconnect 460. As in the example illustrated in FIG. 3, database engine head node 420 may include a client-side storage service driver 425. In this example, distributed database-optimized storage system 410 includes multiple storage system server nodes (including those shown as 430, 440, and 450), each of which includes storage for data pages and redo logs for the segment(s) it stores, and hardware and/or software configured to perform various segment management functions. For example, each storage system server node may include hardware and/or software configured to perform at least a portion of any or all of the following operations: replication (locally, e.g., within the storage node), coalescing of redo logs to generate data pages, snapshots (e.g., creating, restoration, deletion, etc.), log management (e.g., manipulating log records), crash recovery, and/or space management (e.g., for a segment). Each storage system server node may also have multiple attached storage devices (e.g., SSDs) on which data blocks may be stored on behalf of clients (e.g., users, client applications, and/or database service subscribers).

In the example illustrated in FIG. 4, storage system server node 430 includes data page(s) 433, segment redo log(s) 435, segment management functions 437, and attached SSDs 471-478. Again note that the label “SSD” may or may not refer to a solid-state drive, but may more generally refer to a local block storage volume, regardless of its underlying hardware. Similarly, storage system server node 440 includes data page(s) 443, segment redo log(s) 445, segment management functions 447, and attached SSDs 481-488; and storage system server node 450 includes data page(s) 453, segment redo log(s) 455, segment management functions 457, and attached SSDs 491-498.

As previously noted, in some embodiments, a sector is the unit of alignment on an SSD and may be the maximum size on an SSD that can be written without the risk that the write will only be partially completed. For example, the sector size for various solid-state drives and spinning media may be 4 KB. In some embodiments of the distributed database-optimized storage systems described herein, each and every sector may include have a 64-bit (8 byte) CRC at the beginning of the sector, regardless of the higher-level entity of which the sector is a part. In such embodiments, this CRC (which may be validated every time a sector is read from SSD) may be used in detecting corruptions. In some embodiments, each and every sector may also include a “sector type” byte whose value identifies the sector as a log sector, a data sector, or an uninitialized sector. For example, in some embodiments, a sector type byte value of 0 may indicate that the sector is uninitialized.

In some embodiments, each of the storage system server nodes in the distributed database-optimized storage system may implement a set of processes running on the node server's operating system that manage communication with the database engine head node, e.g., to receive redo logs, send back data pages, etc. In some embodiments, all data blocks written to the distributed database-optimized storage system may be backed up to long-term and/or archival storage (e.g., in a remote key-value durable backup storage system).

FIG. 5 is a block diagram illustrating the use of a separate distributed database-optimized storage system in a database system, according to one embodiment. In this example, one or more client processes 510 may store data to one or more databases maintained by a database system that includes a database engine 520 and a distributed database-optimized storage system 530. In the example illustrated in FIG. 5, database engine 520 includes database tier components 560 and client-side driver 540 (which serves as the interface between distributed database-optimized storage system 530 and database tier components 560). In some embodiments, database tier components 560 may perform functions such as those performed by query parsing, optimization and execution component 305 and transaction and consistency management component 330 of FIG. 3, and/or may store data pages, transaction logs and/or undo logs (such as those stored by data page cache 335, transaction log 340 and undo log 345 of FIG. 3).

In this example, one or more client processes 510 may send database query requests 515 (which may include read and/or write requests targeting data stored on one or more of the storage nodes 535 a-535 n) to database tier components 560, and may receive database query responses 517 from database tier components 560 (e.g., responses that include write acknowledgements and/or requested data). Each database query request 515 that includes a request to write to a data page may be parsed and optimized to generate one or more write record requests 541, which may be sent to client-side driver 540 for subsequent routing to distributed database-optimized storage system 530. In this example, client-side driver 540 may generate one or more redo log records 531 corresponding to each write record request 541, and may send them to specific ones of the storage nodes 535 of distributed database-optimized storage system 530. Distributed database-optimized storage system 530 may return a corresponding write acknowledgement 523 for each redo log record 531 to database engine 520 (specifically to client-side driver 540). Client-side driver 540 may pass these write acknowledgements to database tier components 560 (as write responses 542), which may then send corresponding responses (e.g., write acknowledgements) to one or more client processes 510 as one of database query responses 517.

In this example, each database query request 515 that includes a request to read a data page may be parsed and optimized to generate one or more read record requests 543, which may be sent to client-side driver 540 for subsequent routing to distributed database-optimized storage system 530. In this example, client-side driver 540 may send these requests to specific ones of the storage nodes 535 of distributed database-optimized storage system 530, and distributed database-optimized storage system 530 may return the requested data pages 533 to database engine 520 (specifically to client-side driver 540). Client-side driver 540 may send the returned data pages to the database tier components 560 as return data records 544, and database tier components 560 may then send the data pages to one or more client processes 510 as database query responses 517.

In some embodiments, various error and/or data loss messages 534 may be sent from distributed database-optimized storage system 530 to database engine 520 (specifically to client-side driver 540). These messages may be passed from client-side driver 540 to database tier components 560 as error and/or loss reporting messages 545, and then to one or more client processes 510 along with (or instead of) a database query response 517.

In some embodiments, the APIs 531-534 of distributed database-optimized storage system 530 and the APIs 541-545 of client-side driver 540 may expose the functionality of the distributed database-optimized storage system 530 to database engine 520 as if database engine 520 were a client of distributed database-optimized storage system 530. For example, database engine 520 (through client-side driver 540) may write redo log records or request data pages through these APIs to perform (or facilitate the performance of) various operations of the database system implemented by the combination of database engine 520 and distributed database-optimized storage system 530 (e.g., storage, access, change logging, recovery, and/or space management operations). As illustrated in FIG. 5, distributed database-optimized storage system 530 may store data blocks on storage nodes 535 a-535 n, each of which may have multiple attached SSDs. In some embodiments, distributed database-optimized storage system 530 may provide high durability for stored data block through the application of various types of redundancy schemes.

Note that in various embodiments, the API calls and responses between database engine 520 and distributed database-optimized storage system 530 (e.g., APIs 531-534) and/or the API calls and responses between client-side driver 540 and database tier components 560 (e.g., APIs 541-545) in FIG. 5 may be performed over a secure proxy connection (e.g., one managed by a gateway control plane), or may be performed over the public network or, alternatively, over a private channel such as a virtual private network (VPN) connection. These and other APIs to and/or between components of the database systems described herein may be implemented according to different technologies, including, but not limited to, Simple Object Access Protocol (SOAP) technology and Representational state transfer (REST) technology. For example, these APIs may be, but are not necessarily, implemented as SOAP APIs or RESTful APIs. SOAP is a protocol for exchanging information in the context of Web-based services. REST is an architectural style for distributed hypermedia systems. A RESTful API (which may also be referred to as a RESTful web service) is a web service API implemented using HTTP and REST technology. The APIs described herein may in some embodiments be wrapped with client libraries in various languages, including, but not limited to, C, C++, Java, C# and Perl to support integration with database engine 520 and/or distributed database-optimized storage system 530.

As noted above, in some embodiments, the functional components of a database system may be partitioned between those that are performed by the database engine and those that are performed in a separate, distributed, database-optimized storage system. In one specific example, in response to receiving a request from a client process (or a thread thereof) to insert something into a database (e.g., to update a single data block by adding a record to that data block), one or more components of the database engine head node may perform query parsing, optimization, and execution, and may send each portion of the query to a transaction and consistency management component. The transaction and consistency management component may ensure that no other client process (or thread thereof) is trying to modify the same row at the same time. For example, the transaction and consistency management component may be responsible for ensuring that this change is performed atomically, consistently, durably, and in an isolated manner in the database. For example, the transaction and consistency management component may work together with the client-side storage service driver of the database engine head node to generate a redo log record to be sent to one of the nodes in the distributed database-optimized storage service and to send it to the distributed database-optimized storage service (along with other redo logs generated in response to other client requests) in an order and/or with timing that ensures the ACID properties are met for this transaction. Upon receiving the redo log record (which may be considered an “update record” by the storage service), the corresponding storage node may update the data block, and may update a redo log for the data block (e.g., a record of all changes directed to the data block). In some embodiments, the database engine may be responsible for generating an undo log record for this change, and may also be responsible for generating a redo log record for the undo log both of which may be used locally (in the database tier) for ensuring transactionality. However, unlike in traditional database systems, the systems described herein may shift the responsibility for applying changes to data blocks to the storage system (rather than applying them at the database tier and shipping the modified data blocks to the storage system). Moreover, as described herein at FIGS. 8-9B, in various embodiments, system-wide checkpoint may be avoided at the database system along with fast recovery from a database system crash due to various log record operations that may be performed by the storage system as well.

A variety of different allocation models may be implemented for an SSD, in different embodiments. For example, in some embodiments, log entry pages and physical application pages may be allocated from a single heap of pages associated with an SSD device. This approach may have the advantage of leaving the relative amount of storage consumed by log pages and data pages to remain unspecified and to adapt automatically to usage. It may also have the advantage of allowing pages to remain unprepared until they are used, and repurposed at will without preparation. In other embodiments, an allocation model may partition the storage device into separate spaces for log entries and data pages. Once such allocation model is illustrated by the block diagram in FIG. 6 and described below.

FIG. 6 is a block diagram illustrating how data and metadata may be stored on a given storage node (or persistent storage device) of a distributed database-optimized storage system, according to one embodiment. In this example, SSD storage space 600 stores an SSD header and other fixed metadata in the portion of the space labeled 610. It stores log pages in the portion of the space labeled 620, and includes a space labeled 630 that is initialized and reserved for additional log pages. One portion of SSD storage space 600 (shown as 640) is initialized, but unassigned, and another portion of the space (shown as 650) is uninitialized and unassigned. Finally, the portion of SSD storage space 600 labeled 660 stores data pages.

In this example, the first usable log page slot is noted as 615, and the last used log page slot (ephemeral) is noted as 625. The last reserved log page slot is noted as 635, and the last usable log page slot is noted as 645. In this example, the first used data page slot (ephemeral) is noted as 665. In some embodiments, the positions of each of these elements (615, 625, 635, 645, and 665) within SSD storage space 600 may be identified by a respective pointer.

In allocation approach illustrated in FIG. 6, valid log pages may be packed into the beginning of the flat storage space. Holes that open up due to log pages being freed may be reused before additional log page slots farther into the address space are used. For example, in the worst case, the first n log page slots contain valid log data, where n is the largest number of valid log pages that have ever simultaneously existed. In this example, valid data pages may be packed into the end of the flat storage space. Holes that open up due to data pages being freed may be reused before additional data page slots lower in the address space are used. For example, in the worst case, the last m data pages contain valid data, where m is the largest number of valid data pages that have ever simultaneously existed.

In some embodiments, before a log page slot can become part of the potential set of valid log page entries, it may need to be initialized to a value that cannot be confused for a valid future log entry page. This is implicitly true for recycled log page slots, since a retired log page has enough metadata to never be confused for a new valid log page. However, when a storage device is first initialized, or when space is reclaimed that had potentially been used to store application data pages, the log page slots may need to be initialized before they are added to the log page slot pool. In some embodiments, rebalancing/reclaiming log space may be performed as a background task.

In the example illustrated in FIG. 6, the current log page slot pool includes the area between the first usable log page slot (at 615) and the last reserved log page slot (625). In some embodiments, this pool may safely grow up to last usable log page slot (625) without re-initialization of new log page slots (e.g., by persisting an update to the pointer that identifies the last reserved log page slot, 635). In this example, beyond the last usable log page slot (which is identified by pointer 645), the pool may grow up to the first used data page slot (which is identified by pointer 665) by persisting initialized log page slots and persistently updating the pointer for the last usable log page slot (645). In this example, the previously uninitialized and unassigned portion of the SSD storage space 600 shown as 650 may be pressed into service to store log pages. In some embodiments, the current log page slot pool may be shrunk down to the position of the last used log page slot (which is identified by pointer) by persisting an update to the pointer for the last reserved log page slot (635).

In the example illustrated in FIG. 6, the current data page slot pool includes the area between the last usable log page slot (which is identified by pointer 645) and the end of SSD storage space 600. In some embodiments, the data page pool may be safely grown to the position identified by the pointer to the last reserved log page slot (635) by persisting an update to the pointer to the last usable log page slot (645). In this example, the previously initialized, but unassigned portion of the SSD storage space 600 shown as 640 may be pressed into service to store data pages. Beyond this, the pool may be safely grown to the position identified by the pointer to the last used log page slot (625) by persisting updates to the pointers for the last reserved log page slot (635) and the last usable log page slot (645), effectively reassigning the portions of SSD storage space 600 shown as 630 and 640 to store data pages, rather than log pages. In some embodiments, the data page slot pool may be safely shrunk down to the position identified by the pointer to the first used data page slot (665) by initializing additional log page slots and persisting an update to the pointer to the last usable log page slot (645).

In embodiments that employ the allocation approach illustrated in FIG. 6, page sizes for the log page pool and the data page pool may be selected independently, while still facilitating good packing behavior. In such embodiments, there may be no possibility of a valid log page linking to a spoofed log page formed by application data, and it may be possible to distinguish between a corrupted log and a valid log tail that links to an as-yet-unwritten next page. In embodiments that employ the allocation approach illustrated in FIG. 6, at startup, all of the log page slots up to the position identified by the pointer to the last reserved log page slot (635) may be rapidly and sequentially read, and the entire log index may be reconstructed (including inferred linking/ordering). In such embodiments, there may be no need for explicit linking between log pages, since everything can be inferred from LSN sequencing constraints.

In some embodiments, a segment may consist of three main parts (or zones): one that contains a hot log, one that contains a cold log, and one that contains user page data. Zones are not necessarily contiguous regions of an SSD. Rather, they can be interspersed at the granularity of the storage page. In addition, there may be a root page for each segment that stores metadata about the segment and its properties. For example, the root page for a segment may store the user page size for the segment, the number of user pages in the segment, the current beginning/head of the hot log zone (which may be recorded in the form of a flush number), the volume epoch, and/or access control metadata.

In some embodiments, the hot log zone may accept new writes from the client as they are received by the storage node. Both Delta User Log Records (DULRs), which specify a change to a user/data page in the form of a delta from the previous version of the page, and Absolute User Log Records (AULRs), which specify the contents of a complete user/data page, may be written completely into the log. Log records may be added to this zone in approximately the order they are received (e.g., they are not sorted by LSN) and they can span across log pages. The log records may be self-describing, e.g., they may contain an indication of their own size. In some embodiments, no garbage collection is performed in this zone. Instead, space may be reclaimed by truncating from the beginning of the log after all required log records have been copied into the cold log. Log sectors in the hot zone may be annotated with the most recent known unconditional VDL each time a sector is written. Conditional VDL CLRs may be written into the hot zone as they are received, but only the most recently written VDL CLR may be meaningful.

In some embodiments, every time a new log page is written, it may be assigned a flush number. The flush number may be written as part of every sector within each log page. Flush numbers may be used to determine which log page was written later when comparing two log pages. Flush numbers are monotonically increasing and scoped to an SSD (or storage node). For example, a set of monotonically increasing flush numbers is shared between all segments on an SSD (or all segments on a storage node).

In some embodiments, in the cold log zone, log records may be stored in increasing order of their LSNs. In this zone, AULRs may not necessarily store data in-line, depending on their size. For example, if they have large payloads, all or a portion of the payloads may be stored in the data zone and they may point to where their data is stored in the data zone. In some embodiments, log pages in the cold log zone may be written one full page at a time, rather than sector-by-sector. Because log pages in the cold zone are written a full page at a time, any log page in the cold zone for which the flush numbers in all sectors are not identical may be considered to be an incompletely written page and may be ignored. In some embodiments, in the cold log zone, DULRs may be able to span across log pages (up to a maximum of two log pages). However, AULRs may not be able to span log sectors, e.g., so that a coalesce operation will be able to replace a DULR with an AULR in a single atomic write.

In some embodiments, the cold log zone is populated by copying log records from the hot log zone. In such embodiments, only log records whose LSN is less than or equal to the current unconditional volume durable LSN (VDL) may be eligible to be copied to the cold log zone. When moving log records from the hot log zone to the cold log zone, some log records (such as many CLRs) may not need to be copied because they are no longer necessary. In addition, some additional coalescing of user pages may be performed at this point, which may reduce the amount of copying required. In some embodiments, once a given hot zone log page has been completely written and is no longer the newest hot zone log page, and all ULRs on the hot zone log page have been successfully copied to the cold log zone, the hot zone log page may be freed and reused.

In some embodiments, garbage collection may be done in the cold log zone to reclaim space occupied by obsolete log records, e.g., log records that no longer need to be stored in the SSDs of the storage tier. For example, a log record may become obsolete when there is a subsequent AULR for the same user page and the version of the user page represented by the log record is not needed for retention on SSD. In some embodiments, a garbage collection process may reclaim space by merging two or more adjacent log pages and replacing them with fewer new log pages containing all of the non-obsolete log records from the log pages that they are replacing. The new log pages may be assigned new flush numbers that are larger than the flush numbers of the log pages they are replacing. After the write of these new log pages is complete, the replaced log pages may be added to the free page pool. Note that in some embodiments, there may not be any explicit chaining of log pages using any pointers. Instead, the sequence of log pages may be implicitly determined by the flush numbers on those pages. Whenever multiple copies of a log record are found, the log record present in the log page with highest flush number may be considered to be valid and the others may be considered to be obsolete.

In some embodiments, e.g., because the granularity of space managed within a data zone (sector) may be different from the granularity outside the data zone (storage page), there may be some fragmentation. In some embodiments, to keep this fragmentation under control, the system may keep track of the number of sectors used by each data page, may preferentially allocate from almost-full data pages, and may preferentially garbage collect almost-empty data pages (which may require moving data to a new location if it is still relevant). Note that pages allocated to a segment may in some embodiments be repurposed among the three zones. For example, when a page that was allocated to a segment is freed, it may remain associated with that segment for some period of time and may subsequently be used in any of the three zones of that segment. The sector header of every sector may indicate the zone to which the sector belongs. Once all sectors in a page are free, the page may be returned to a common free storage page pool that is shared across zones. This free storage page sharing may in some embodiments reduce (or avoid) fragmentation.

In some embodiments, the distributed database-optimized storage systems described herein may maintain various data structures in memory. For example, for each user page present in a segment, a user page table may store a bit indicating whether or not this user page is “cleared” (i.e., whether it includes all zeroes), the LSN of the latest log record from the cold log zone for the page, and an array/list of locations of all log records from the hot log zone for page. For each log record, the user page table may store the sector number, the offset of the log record within that sector, the number of sectors to read within that log page, the sector number of a second log page (if the log record spans log pages), and the number of sectors to read within that log page. In some embodiments, the user page table may also store the LSNs of every log record from the cold log zone and/or an array of sector numbers for the payload of the latest AULR if it is in the cold log zone.

In some embodiments of the distributed database-optimized storage systems described herein, an LSN index may be stored in memory. An LSN index may map LSNs to log pages within the cold log zone. Given that log records in cold log zone are sorted, it may be to include one entry per log page. However, in some embodiments, every non-obsolete LSN may be stored in the index and mapped to the corresponding sector numbers, offsets, and numbers of sectors for each log record.

In some embodiments of the distributed database-optimized storage systems described herein, a log page table may be stored in memory, and the log page table may be used during garbage collection of the cold log zone. For example, the log page table may identify which log records are obsolete (e.g., which log records can be garbage collected) and how much free space is available on each log page.

In the storage systems described herein, an extent may be a logical concept representing a highly durable unit of storage that can be combined with other extents (either concatenated or striped) to represent a volume. Each extent may be made durable by membership in a single protection group. An extent may provide an LSN-type read/write interface for a contiguous byte sub-range having a fixed size that is defined at creation. Read/write operations to an extent may be mapped into one or more appropriate segment read/write operations by the containing protection group. As used herein, the term “volume extent” may refer to an extent that is used to represent a specific sub-range of bytes within a volume.

As noted above, a volume may consist of multiple extents, each represented by a protection group consisting of one or more segments. In some embodiments, log records directed to different extents may have interleaved LSNs. For changes to the volume to be durable up to a particular LSN it may be necessary for all log records up to that LSN to be durable, regardless of the extent to which they belong. In some embodiments, the client may keep track of outstanding log records that have not yet been made durable, and once all ULRs up to a specific LSN are made durable, it may send a Volume Durable LSN (VDL) message to one of the protection groups in the volume. The VDL may be written to all synchronous mirror segments for the protection group. This is sometimes referred to as an “Unconditional VDL” and it may be periodically persisted to various segments (or more specifically, to various protection groups) along with write activity happening on the segments. In some embodiments, the Unconditional VDL may be stored in log sector headers.

In various embodiments, the operations that may be performed on a segment may include writing a DULR or AULR received from a client (which may involve writing the DULR or AULR to the tail of the hot log zone and then updating the user page table), reading a cold user page (which may involve locating the data sectors of the user page and returning them without needing to apply any additional DULRs), reading a hot user page (which may involve locating the data sectors of the most recent AULR for the user page and apply any subsequent DULRs to the user page before returning it), replacing DULRs with AULRs (which may involve coalescing DULRs for a user page to create an AULR that replaces the last DULR that was applied), manipulating the log records, etc. As described herein coalescing is the process of applying DULRs to an earlier version of a user page to create a later version of the user page. Coalescing a user page may help reduce read latency because (until another DULR is written) all DULRs written prior to coalescing may not need to be read and applied on demand. It may also help reclaim storage space by making old AULRs and DULRs obsolete (provided there is no snapshot requiring the log records to be present). In some embodiments, a coalescing operation may include locating a most recent AULR and applying any subsequent DULRs in sequence without skipping any of the DULRs. As noted above, in some embodiments, coalescing may not be performed within the hot log zone. Instead, it may be performed within the cold log zone. In some embodiments, coalescing may also be performed as log records are copied from the hot log zone to the cold log zone.

In some embodiments, the decision to coalesce a user page may be triggered by the size of the pending DULR chain for the page (e.g., if the length of the DULR chain exceeds a pre-defined threshold for a coalescing operation, according to a system-wide, application-specific or client-specified policy)), or by the user page being read by a client.

Commonly, database systems persist modifications or changes to data stored for a database by persisting multiple copies of data/records/or other parts of transaction information to different locations in persistent storage system. For example, a received update to a database may require writing one or more redo records to a redo log on disk and writing the update described by the one or more redo log to the location storing the physical data on disk. A record for an undo log, to ensure read consistency, may also be written to disk. Finally, an update to a system transaction table may also be written to disk. Thus, when processing changes to databases for example, typical database systems must perform significant IOPS (Input/Output Operations Per Second) to simply ensure the persistence of updates to the database. These data systems are but one example of log-structured storage systems that may often require separately persisting log records.

However, as described below, various methods and techniques may be used reduce transaction I/O. FIG. 7 is a block diagram illustrating a method to reduce transaction I/O for a log-structured storage system, according to some embodiments. Many of the examples below may be performed using various embodiments of a distributed database system described above with respect to FIGS. 1-6. Various components of the systems described above, such as a component of a storage system server node 430, may be configured to perform the various techniques described with regard to FIG. 7. However, other configurations of distributed storage, database systems, or other log-structured storage system generally, such as one of the various components of computer system 1000 described below with regard to FIG. 11, may also implement the techniques described herein to reduce I/O transactions, and as such, the following discussion is not to be construed as limiting to any one of the multiple examples given. For example, a log-structured storage system may, in some embodiments, be implemented as part of a file system. Various changes, updates, or manipulations to the files of the file system may be persisted as log records describing changes to various pages for data (or metadata) of the file system.

In various embodiments, a plurality of log records for a plurality of pages for file-structured storage may be stored in a persistent storage device, as indicated at 710. As discussed above with regard to FIG. 5 numerous log records, such as the various illustrated redo log records, may be generated in response to various access requests to a database. These log records may be stored on one of many different types of persistent data storage structures, such as the SSDs illustrated in FIG. 4, and maintained or arranged in a variety of different data structures and/or formats. For example, in at least some embodiments log records, such as redo records (or redo records that correspond to undo logs or system transaction tables) may be maintained in a log data structure or other arrangement, such as SSD storage space 600 discussed above with regard to FIG. 6. Similarly, in various other embodiments, changes to one or more items persisted in log storage, such as to a file in a file system, may be represented as log records of various types in a storage space.

As discussed above, in various embodiments log records may be one of many different log record types. For example, a redo log record type may be implemented that describes various changes, modifications, or updates to a data page for a database. Redo log records may be implemented as part of a redo log which, may, in the event of system failure or crash, be replayed to generate a current state of the database. Redo log records, such as the User Log Records (ULRs) or the Absolute User Log Records (AULRs) discussed above, may also describe these various changes, modifications, or updates to a data page, dependent or not dependent on a prior version of the data page (or log record). A redo record, such as an ULR, may describe an update to a data page that, for instance, increments the current value by 1. A redo record, such as an AULR, may describe an update to a data page to set the value of the data page to 253. Redo log records may also specify that a particular portion or byte range of a data page may be changed, modified, or updated. For instance, in some embodiments, the redo record may update a portion of a data page that represents a counter. Such an update may be made by a redo record that is dependent or not dependent on the prior version of the portion of the data page to generate the current version of the data page (similar to the examples discussed above with regard to the ULR and AULR examples. In at least some embodiments, redo log records may describe or indicate an overwrite of a previous version of a data page or portion of the data page corresponding to the redo log record. For instance, the data page may store a counter value, and the redo log record may overwrite the previous counter value with a new counter value. Numerous other examples of overwrites, such that a new value of a page replaces, modifies, or is independent of a previous value of data page may envisioned, and as such, the previous example is not intended to be limiting.

In some embodiments, redo log records may also be used to describe changes to data pages for an undo log, transaction table, or some other type or form data structure that is not user data for the database. For example, redo records may record the various entries for an undo to log, such that in the event a determination is made to undo one or more changes to a data page, the redo log records for the undo log may be replayed to remove the changes made. FIG. 9A described in more detail below provides a more detailed example. Similarly, in another example, redo records may be used to describe a change or entry to a transaction table. For instance, in some embodiments, each transaction (such as a system transaction) may be associated with a larger series of system transactions (such as a user transaction). An entry for each system transaction may be maintained as a different page. A redo record may describe the status of the system transaction for the transaction page of the respective entry of the system transaction in the transaction table. FIG. 9B described in more detail below provides a more detailed example with regard to transaction tables. In some embodiments, log records may be redo log records of specific types such as an undo record or a transaction table record. However, in at least some embodiments, log records may be an undo log record type or a transaction table log record type, with a similar or different format than the redo records.

Other log record types may, in some embodiments, describe other changes, modifications, manipulations, to pages stored for a system implementing log-structure storage. Various types of log records for file information or metadata, including, but not limited to, file size, last time modified, etc., may be used for a file system, for instance.

A coalesce operation may be performed for at least some of the plurality of pages of the log-structured storage, as indicated at 720. This coalesce operation may separately store a new instance or version of the data page based on log records. A new instance or version of the page may, in some embodiments be a new type of log record stored in the log, such as an AULR described above. This new log record stored in the log may be dependent (e.g., DULR), or independent (e.g., AULR). A new instance or version of the page may, in some embodiments, be a new page stored in a data zone, such as described in FIG. 6. FIG. 8 is a high-level flow chart of a method to perform a coalesce operation, according to some embodiments. A coalesce event or some other indication may be received, detected, or determined, that indicates that coalesce operation for a particular page may be performed. For instance, in some embodiments, log records for a page may be determined to exceed a coalesce operation threshold for the page, as indicated at 810. For example, in some embodiments, a coalesce operation counter may be maintained for each page that indicates a number of log records for the page. Thus, for instance, the coalesce operation counter may indicate that log records for the page exceed a coalesce operation threshold for the page. A coalesce operation threshold may be specific to individual data pages, or types of data pages, in some embodiments. For instance, a coalesce operation threshold for log records for a transaction table entry may be lower than a coalesce operation threshold for an pages for an undo log or redo log for a page. In some embodiments, a coalesce operation threshold may be system-wide and, thus, the same for all pages of a database.

In some embodiments, after determining to perform a coalesce operation, two or more of the log records for the page upon which the coalesce operation is performed may be coalesced to generate an instance of the page, as indicated at 820. A coalesce operation, as described above, may combine two more log records. These log records may, for example, describe that the value of the page is 253 and that the value of the page is to be increased by 1. The generated instance of the page may be to combine these two records and create an instance of the page's value at 254. The generated instance of the page may be stored to a new location in persistent storage, as indicated 830. In at least some embodiments, the log records used to generate the new instance may remain in persistent storage until some form of garbage collection or other restructuring process removes the records from the persistent storage.

A coalesce operation may be performed as a background process, in some embodiments. Various foreground processes, such as responding to access requests, may be performed with higher frequency or priority before a coalesce operation. Thus, for example, a period of time may occur in between determining that the coalesce operation threshold for page is exceeded, as indicated at 810, and the coalesce operation is performed, at 820 and 830. The method described above with regard to FIG. 8 may be performed repeatedly for multiple different pages.

In at least some embodiments, log records for one or more of the plurality of pages may be excluded from the coalesce operation, as indicated at 730. Each of these excluded log records may not be dependent on a prior version of the page or portion of the page corresponding to the log record. As noted in the example above, log records may be dependent on a prior version of a page or portion of a page to describe a change, modification, or update to that page. If, for instance, a log record describes that the value of the page is to be incremented by 1, then the log record is dependent on the previous value of the page to be incremented so that the new value of the page may be known. Alternatively, log record may not be dependent on a prior version of the page or portion of the page. For example, a log record may describe the value of the page is to be changed to 200.

A log record may be analyzed or evaluated to determine whether a log record is dependent on a prior version of a page or a portion of the page corresponding to the log record. For instance, the log record may be read, scanned, parsed, examined for relative instructions, such as increment, decrement, add, multiply, divide, or some other formula or manipulation to be applied. Alternatively, single values or the size of the record may, for example, be used to determine that the log record independently describes the value of the page. In at least some embodiments, the determination as to the dependency, or lack thereof, on prior versions or portions of corresponding pages based on the type of log record. For example, an undo log record, or a transaction table record may automatically be identified and determined to lack dependency. In some embodiments, a bit, field, signature, or some value or header information included in or along with the log record may indicate dependency on prior versions of pages. For example, in some embodiments, a database head node, such as database head node 420 described above in FIG. 4, may set a bit in log records that are generated and sent to storage nodes, to indicate which log records are dependent or independent on previous pages or portions of pages.

In some embodiments, excluding the log records from the coalesce operation may be performed in various ways. For examples, various metadata or data structures may be maintained which list whether or not a log record is eligible for the coalesce operation. In some embodiments, as noted above, a coalesce operation counter may be maintained. When a log record that is determined not to be dependent on a prior version of a data page or portion of the data page corresponding to the log record, the coalesce counter may be reset. For example, in some embodiments, the coalesce counter may be reset to a value of 1. Various other manipulations to a coalesce counter (or other record keeping structure) may be made. For instance, in some embodiments, for log records that lack dependency, the coalesce operation counter may not be altered when the record is stored. Thus, for instance, if the coalesce operation counter currently indicates that 5 log records are stored a particular data page, and an additional log record that is independent is stored, then the counter may be maintained at 5. Alternatively, if the log record only indicates a change to portion of a page, such as byte, or range of bytes, in a page, then counter may be fractionally incremented, such as by 0.5, or some other value proportional to the change to the page. Please note, that the examples given above are not intended to be limiting as to the various methods and techniques for excluding log records from a coalesce operation.

The types of log records maintained at the persistent storage device, and/or excluded from a coalesce operation may vary. Redo records, for instance, may in some embodiments be determined to be independent from previous versions of the page or portion of the page corresponding to the redo record. Undo records and transaction table records (or redo records that represent them) may also be excluded from a coalesce operation. FIG. 9A is a block diagram illustrating examples of reduced transaction I/O for log records for an undo log of a database, according to some embodiments. Undo log 900 may be implemented as multiple pages. In this illustrated example, pages 902, 904, 906, and 908 may illustrate pages implementing the undo log. As log records for the undo log pages are received and stored, the logical structure of the log may be maintained by identifying each newly received entry as located at the end of the log. Arrow 910 illustrates that each new entry traverses from the beginning of the log at page 902 toward the end of the log at page 908. In at least some embodiments, undo log 900 is implemented as a circular log. Thus, arrow 920 illustrates that upon reaching the end of the undo log at page 908, a newly received entry may identified as located at the beginning of the log in page 902.

In at least some embodiments, a coalesce operation threshold may be set for each of the data pages 902-908. This coalesce operation threshold may be determined based on the number log records that may be stored in a page for the undo log. In various embodiments, the coalesce operation threshold may be set higher than the number of log records that can be stored in a page for the undo log. Thus, coalesce operation thresholds 903, 905, 907, and 909, allow, in the illustrated example, the 4 log records for the undo page that can be written without exceeding the threshold. Implementing the techniques described above with regard to FIGS. 7 and 8, a log record for the undo log that starts over (as directed by arrow 920) at the beginning of page 902 may indicate (or be determined to indicate) that it is not dependent on the previous version of undo page 902 (or portion of undo page 902). Thus, a coalesce operation counter for page 902, would not increase from its current number 4 (indicating the number of log records for the undo page) to 5, but instead may be reset to 1. Similarly, when coming again to pages 904, 906, and 908, the coalesce operation counter may be reset to 1 such that the number of records indicated for each of the undo pages does not exceed their respective coalesce operation thresholds, 905, 907, and 909. In this way, the log records for the undo log may be maintained as previously stored without storing a new instance of the undo pages to a new location in persistent storage.

As noted above, transaction table records may also implement the various techniques and methods described above with regard to FIGS. 7 and 8. FIG. 9B is a block diagram illustrating examples of reduced transaction I/O for transaction table records for a transaction table of database, according to some embodiments. In various embodiments, a transaction table may be maintained for transactions that are performed for a database. A transaction table 930 may provide entries for various user transactions, such as user transaction 940 and 950, as well as system transactions associated with those user transactions, 942, 944, 946, 948, 952, 954, 956, and 958. A user transaction, such as 940 and 950, may describe a user instruction or request with respect to a database, such as create a new row or column for this table filled with certain values. A system transaction may, in some embodiments, be a discrete or atomic operation that is used to implement the user transaction. Multiple system transactions may be used to implement a user transaction. For each system transaction, transaction table 930 may maintain a current status of the transaction, acknowledge, committed, rolled-back, etc. In at least some embodiments, each system transaction may have a respective system transaction page which indicates the current status of the transaction. For example, system transaction 942 may indicate the current status of the system transaction table entry that corresponds to system transaction page 942.

In at least some embodiments log records, such as transaction table records (or redo records describing the transaction table page) may be stored for a respective system transaction page, such as system transaction page 942. A new log record for system transaction page 942 may be received and stored. In various embodiments, a system transaction may have only one status at a time, and therefore, the new log record for the system transaction record may not be dependent on the prior version of system transaction page 942. The new log record may indicate this lack of dependency or it may be determined or known that a transaction table record lacks dependency, and maintained without separately storing a new instance of the system transaction page 942. In some embodiments, upon receipt and storage of the new log record for system transaction page 942, a coalesce operation counter for page 942 may be reset, such that a coalesce operation threshold for system transaction page 942 is not exceeded, thus excluding the log record from the coalesce operation. Similar techniques may be applied to log records for other system transaction pages in transaction table 930.

Please note that FIGS. 9A and 9B represent logical illustrations of log records and pages. For example, in FIG. 9A, the log records stored in persistent store may be a part of a larger log record store, such as SSD 600 in FIG. 6, and thus may not physically or logically be arranged in the order in the persistent storage device as illustrated. Likewise, the transaction table records may be stored in the persistent storage device as part of a log structure and thus interleaved with other log records of other log records types. Similarly, the pages depicted in FIGS. 9A and 9B may also not be contiguously stored, either physically or logically, but may, in some embodiments be arranged according to various schemes, such as SSD 600 discussed above with FIG. 6. As such the previous examples, explanations, and illustrations, are not intended to be limiting as to other embodiments.

FIG. 10 is a flow diagram illustrating a method to perform reduced transaction I/O for an update to a database, according to some embodiments. The various techniques and examples given above with regard to FIGS. 7-9B may also be used in conjunction with the following techniques. Although the previous examples were given in the context of a log-structured storage system, FIG. 10 may describe how techniques of reducing transaction I/O may be applied to a non-distributed database or single service architecture database in addition to the architectures described above with regard to FIGS. 1-6.

As indicated at 1110, an update for a database may be received that is part of a transaction. This update may be a change, modification, or manipulation of data for a database. The transaction may be a user transaction, such as the user transactions discussed above with regard to FIG. 9B, or some other type of transaction. A plurality of log records including a redo log record for a data page modified by the update, a log record for an undo page modified for the update, and a log record updating a transaction table page, may be generated, as indicated at 1120. These log records may be generated in response to the received update, and in some embodiments may be sent to a storage node of a distributed storage service, such as illustrated in FIG. 4.

In various embodiments, a storage operation may be performed to persistently store the plurality of log records, as indicated at 1130. These log records may be persisted in a log structure or other data store, such as SSD 600 described above in FIG. 6. As noted above, various storage operations, such as a coalesce operation, may be performed as part of a background process, in some embodiments. Therefore, a period of time may pass before elements 1140, 1150, and 1160 are performed related to system availability or other balancing between foreground and background processing. Subsequent to persistently storing the plurality of log records, the redo log record for the data page modified by the update for the data page may be combined with one or more redo log records for the data page to create an instance of the data page, as indicated at 1140. In various embodiments, this combination may be a coalesce operation or other type of operation to represent the combined records as an instance of a single page. The combination may be performed in response to exceeding a storage operation threshold, using techniques similar to those described above with regard to FIG. 8. Then, a storage operation to persistently store the instance of the data page separate from the plurality of log records, may be performed, as indicated at 1150. For example, SSD 600 in FIG. 6 illustrates that log records may be stored on log pages separate from data pages.

As indicated at 1160, the undo page and the transaction table page may be maintained as log records, in some embodiments, without performing persistent storage operations to store the undo page and the transaction table as separate pages, such as in data pages in SSD 600 in FIG. 6. The log record for the undo page may not be dependent on a prior version of the undo page. Additionally, the log record for the transaction table page may not be dependent on the prior version of the transaction table page. Determining whether the log records for the undo page and the transaction table page are dependent may be made using the various techniques discussed above with regard to FIG. 7. The various techniques described above with regard to FIGS. 7, 8, and 9A may be implemented to maintain the log record for the undo page without performing a separate storage operation. Similarly, the various techniques described above with regard to FIGS. 7, 8, and 9B may be implemented to maintain the log record for the transaction table page without performing as separate storage operation.

Please note, that previously discussed flow charts and figures are not intended to be limiting as to any particular ordering of the elements. Various determinations or operations may be performed in different orderings, in a parallel, dynamically, or in response to various other indications and/or determinations. Additional elements or steps may be added, or other elements removed without limiting the various techniques described to reduce transaction I/O for distributed database systems.

FIG. 11 is a block diagram illustrating a computer system configured to implement at least a portion of the database systems described herein, according to various embodiments. For example, computer system 1000 may be configured to implement a database engine head node of a database tier, or one of a plurality of storage nodes of a separate distributed database-optimized storage system that stores databases and associated metadata on behalf of clients of the database tier, in different embodiments. Computer system 1000 may also be configured to implement a database system that includes both the database tier and the storage tier. Computer system 1000 may be any of various types of devices, including, but not limited to, a personal computer system, desktop computer, laptop or notebook computer, mainframe computer system, handheld computer, workstation, network computer, a consumer device, application server, storage device, telephone, mobile telephone, or in general any type of computing device.

Computer system 1000 includes one or more processors 1010 (any of which may include multiple cores, which may be single or multi-threaded) coupled to a system memory 1020 via an input/output (I/O) interface 1030. Computer system 1000 further includes a network interface 1040 coupled to I/O interface 1030. In various embodiments, computer system 1000 may be a uniprocessor system including one processor 1010, or a multiprocessor system including several processors 1010 (e.g., two, four, eight, or another suitable number). Processors 1010 may be any suitable processors capable of executing instructions. For example, in various embodiments, processors 1010 may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each of processors 1010 may commonly, but not necessarily, implement the same ISA. The computer system 1000 also includes one or more network communication devices (e.g., network interface 1040) for communicating with other systems and/or components over a communications network (e.g. Internet, LAN, etc.). For example, a client application executing on system 1000 may use network interface 1040 to communicate with a server application executing on a single server or on a cluster of servers that implement one or more of the components of the database systems described herein. In another example, an instance of a server application executing on computer system 1000 may use network interface 1040 to communicate with other instances of the server application (or another server application) that may be implemented on other computer systems (e.g., computer systems 1090).

In the illustrated embodiment, computer system 1000 also includes one or more persistent storage devices 1060 and/or one or more I/O devices 1080. In various embodiments, persistent storage devices 1060 may correspond to disk drives, tape drives, solid state memory, other mass storage devices, or any other persistent storage device. Computer system 1000 (or a distributed application or operating system operating thereon) may store instructions and/or data in persistent storage devices 1060, as desired, and may retrieve the stored instruction and/or data as needed. For example, in some embodiments, computer system 1000 may host a storage system server node, and persistent storage 1060 may include the SSDs attached to that server node.

Computer system 1000 includes one or more system memories 1020 that are configured to store instructions and data accessible by processor(s) 1010. In various embodiments, system memories 1020 may be implemented using any suitable memory technology, (e.g., one or more of cache, static random access memory (SRAM), DRAM, RDRAM, EDO RAM, DDR 10 RAM, synchronous dynamic RAM (SDRAM), Rambus RAM, EEPROM, non-volatile/Flash-type memory, or any other type of memory). System memory 1020 may contain program instructions 1025 that are executable by processor(s) 1010 to implement the methods and techniques described herein. In various embodiments, program instructions 1025 may be encoded in platform native binary, any interpreted language such as Java™ byte-code, or in any other language such as C/C++, Java™, etc., or in any combination thereof. For example, in the illustrated embodiment, program instructions 1025 include program instructions executable to implement the functionality of a database engine head node of a database tier, or one of a plurality of storage nodes of a separate distributed database-optimized storage system that stores databases and associated metadata on behalf of clients of the database tier, in different embodiments. In some embodiments, program instructions 1025 may implement multiple separate clients, server nodes, and/or other components.

In some embodiments, program instructions 1025 may include instructions executable to implement an operating system (not shown), which may be any of various operating systems, such as UNIX, LINUX, Solaris™, MacOS™, Windows™, etc. Any or all of program instructions 1025 may be provided as a computer program product, or software, that may include a non-transitory computer-readable storage medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to various embodiments. A non-transitory computer-readable storage medium may include any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). Generally speaking, a non-transitory computer-accessible medium may include computer-readable storage media or memory media such as magnetic or optical media, e.g., disk or DVD/CD-ROM coupled to computer system 1000 via I/O interface 1030. A non-transitory computer-readable storage medium may also include any volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc., that may be included in some embodiments of computer system 1000 as system memory 1020 or another type of memory. In other embodiments, program instructions may be communicated using optical, acoustical or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, etc.) conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via network interface 1040.

In some embodiments, system memory 1020 may include data store 1045, which may be configured as described herein. For example, the information described herein as being stored by the database tier (e.g., on a database engine head node), such as a transaction log, an undo log, cached page data, or other information used in performing the functions of the database tiers described herein may be stored in data store 1045 or in another portion of system memory 1020 on one or more nodes, in persistent storage 1060, and/or on one or more remote storage devices 1070, at different times and in various embodiments. Similarly, the information described herein as being stored by the storage tier (e.g., redo log records, coalesced data pages, and/or other information used in performing the functions of the distributed storage systems described herein) may be stored in data store 1045 or in another portion of system memory 1020 on one or more nodes, in persistent storage 1060, and/or on one or more remote storage devices 1070, at different times and in various embodiments. In general, system memory 1020 (e.g., data store 1045 within system memory 1020), persistent storage 1060, and/or remote storage 1070 may store data blocks, replicas of data blocks, metadata associated with data blocks and/or their state, database configuration information, and/or any other information usable in implementing the methods and techniques described herein.

In one embodiment, I/O interface 1030 may be configured to coordinate I/O traffic between processor 1010, system memory 1020 and any peripheral devices in the system, including through network interface 1040 or other peripheral interfaces. In some embodiments, I/O interface 1030 may perform any necessary protocol, timing or other data transformations to convert data signals from one component (e.g., system memory 1020) into a format suitable for use by another component (e.g., processor 1010). In some embodiments, I/O interface 1030 may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some embodiments, the function of I/O interface 1030 may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some embodiments, some or all of the functionality of I/O interface 1030, such as an interface to system memory 1020, may be incorporated directly into processor 1010.

Network interface 1040 may be configured to allow data to be exchanged between computer system 1000 and other devices attached to a network, such as other computer systems 1090 (which may implement one or more storage system server nodes, database engine head nodes, and/or clients of the database systems described herein), for example. In addition, network interface 1040 may be configured to allow communication between computer system 1000 and various I/O devices 1050 and/or remote storage 1070. Input/output devices 1050 may, in some embodiments, include one or more display terminals, keyboards, keypads, touchpads, scanning devices, voice or optical recognition devices, or any other devices suitable for entering or retrieving data by one or more computer systems 1000. Multiple input/output devices 1050 may be present in computer system 1000 or may be distributed on various nodes of a distributed system that includes computer system 1000. In some embodiments, similar input/output devices may be separate from computer system 1000 and may interact with one or more nodes of a distributed system that includes computer system 1000 through a wired or wireless connection, such as over network interface 1040. Network interface 1040 may commonly support one or more wireless networking protocols (e.g., Wi-Fi/IEEE 802.11, or another wireless networking standard). However, in various embodiments, network interface 1040 may support communication via any suitable wired or wireless general data networks, such as other types of Ethernet networks, for example. Additionally, network interface 1040 may support communication via telecommunications/telephony networks such as analog voice networks or digital fiber communications networks, via storage area networks such as Fibre Channel SANs, or via any other suitable type of network and/or protocol. In various embodiments, computer system 1000 may include more, fewer, or different components than those illustrated in FIG. 11 (e.g., displays, video cards, audio cards, peripheral devices, other network interfaces such as an ATM interface, an Ethernet interface, a Frame Relay interface, etc.)

It is noted that any of the distributed system embodiments described herein, or any of their components, may be implemented as one or more web services. For example, a database engine head node within the database tier of a database system may present database services and/or other types of data storage services that employ the distributed storage systems described herein to clients as web services. In some embodiments, a web service may be implemented by a software and/or hardware system designed to support interoperable machine-to-machine interaction over a network. A web service may have an interface described in a machine-processable format, such as the Web Services Description Language (WSDL). Other systems may interact with the web service in a manner prescribed by the description of the web service's interface. For example, the web service may define various operations that other systems may invoke, and may define a particular application programming interface (API) to which other systems may be expected to conform when requesting the various operations.

In various embodiments, a web service may be requested or invoked through the use of a message that includes parameters and/or data associated with the web services request. Such a message may be formatted according to a particular markup language such as Extensible Markup Language (XML), and/or may be encapsulated using a protocol such as Simple Object Access Protocol (SOAP). To perform a web services request, a web services client may assemble a message including the request and convey the message to an addressable endpoint (e.g., a Uniform Resource Locator (URL)) corresponding to the web service, using an Internet-based application layer transfer protocol such as Hypertext Transfer Protocol (HTTP).

In some embodiments, web services may be implemented using Representational State Transfer (“RESTful”) techniques rather than message-based techniques. For example, a web service implemented according to a RESTful technique may be invoked through parameters included within an HTTP method such as PUT, GET, or DELETE, rather than encapsulated within a SOAP message.

The various methods as illustrated in the figures and described herein represent example embodiments of methods. The methods may be implemented manually, in software, in hardware, or in a combination thereof. The order of any method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc.

Although the embodiments above have been described in considerable detail, numerous variations and modifications may be made as would become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such modifications and changes and, accordingly, the above description to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. A system, comprising: at least one hardware processor; a hardware memory comprising program instructions that when executed by the at least one hardware processor cause the at least one hardware processor to implement a storage node of a distributed storage service; the storage node, configured to: store, in persistent storage, a plurality of log records indicating updates to respective pages of a plurality of pages of a database; determine, from the plurality of log records, a first subset of one or more log records for one or more of the plurality of pages that indicate one or more respective updates that are not dependent on data of a prior version of the page or portion of the page corresponding to the respective log record to produce respective results of the updates; exclude, from a coalesce operation, the first subset of one or more log records for the one or more pages; and perform the coalesce operation for at least some of the plurality of pages of the database, wherein to perform the coalesce operation, the storage node is configured to: apply updates indicated by a second subset of two or more of the plurality of log records that are not excluded for one of the at least some of the plurality of pages to a previous instance of the page to generate a new instance of the page; and store the instance of the page to a new location in the persistent storage; and subsequent to the coalesce operation, remove the second subset of log records whose updates were applied from the persistent storage and retain the first subset of log records in the persistent storage.
 2. The system of claim 1, wherein the storage node is further configured to determine that log records for the at least some of the plurality of pages of the database exceed a respective coalesce operation threshold for the page.
 3. The system of claim 2, wherein a coalesce operation counter is maintained for each of the plurality of pages to indicate a number of log records for the page, and wherein, to exclude, from the coalesce operation, the one or more log records for the one or more pages, the storage node is configured to: reset the coalesce operation counter for each of the one or more pages.
 4. The system of claim 1, wherein the one or more log records for the one or more pages that are excluded from the coalesce operation are one or more redo records, one or more undo records, or one or more transaction table records.
 5. The system of claim 1, wherein the system further comprises a database engine node configured to: generate the plurality of log records for the plurality of pages of the database in response to one or more updates to the database, wherein said generation comprises including an identifier in the log record which indicates whether the log record is dependent on data of a prior version of a page or a portion of the page corresponding to the log record; and send the plurality of log records to the storage node.
 6. A method, comprising: performing, by one or more computing devices: storing, in persistent storage, a plurality of log records indicating updates to respective pages of a plurality of pages of log-structured storage; determining, from the plurality of log records, a first subset of one or more of the plurality of log records for one or more of the plurality of pages that indicate one or more respective updates that are not dependent on data of a prior version of the page or portion of the page corresponding to the respective log record to produce respective results of the updates; excluding, from a coalesce operation, the first subset of one or more log records for the one or more pages; and performing the coalesce operation for at least some of the plurality of pages of the log-structured storage, wherein said performing comprises: applying updates indicated in a second subset of two or more of the plurality of log records that are not excluded for one of the at least some of the plurality of pages to a previous instance of the page to generate a new instance of the page; and storing the instance of the page to a new location in the persistent storage; and subsequent to the coalesce operation, removing the second subset of log records whose updates were applied from the persistent storage and retaining the first subset of log records in the persistent storage.
 7. The method of claim 6, wherein said determining comprises evaluating a dependency indication included in each log record of the plurality of log records, wherein the dependency indications for the one or more log records indicate a lack of dependency on the data of the prior version of the page or portion of the page corresponding to the log record.
 8. The method of claim 6, wherein said determining is based, at least in part, on a log record type for the one or more log records.
 9. The method of claim 6, further comprising determining that the log records for the at least some of the plurality of pages of the log-structured storage exceed a respective coalesce operation threshold for the page.
 10. The method of claim 9, wherein a coalesce operation counter is maintained for each of the plurality of pages to indicate a number of log records for the page, and wherein said excluding, from the coalesce operation, the one or more log records for the one or more pages, comprises: resetting the coalesce operation counter for each of the one or more pages for which the log record is not dependent on the data of the prior version of the page or the portion of the page corresponding to the log record.
 11. The method of claim 9, wherein the coalesce operation threshold for each of the plurality of pages is based, at least in part, on a number of log records that may be stored on the page.
 12. The method of claim 6, wherein the log-structured storage is implemented as part of a file system managing one or more files, and wherein the one or more log records excluded from the coalesce operation include one or more updates to metadata for the one or more files.
 13. The method of claim 6, wherein the log-structured storage is implemented as storage for a database.
 14. The method of claim 13, wherein the one or more log records excluded from the coalesce operation include one or more undo records.
 15. The method of claim 13, wherein the one or more log records excluded from the coalesce operation include one or more transaction table records.
 16. The method of claim 13, wherein the one or more log records excluded from the coalesce operation include one or more redo records.
 17. The method of claim 16, wherein the one or more redo records indicate an overwrite of the prior version of the page or the portion of the page corresponding to the redo record.
 18. The method of claim 13, wherein the one or more computing devices together implement a storage node of a distributed storage service, wherein the plurality of log records are received from a database service implementing the database.
 19. A non-transitory, computer-readable storage medium, storing program instructions that when executed by one or more computing devices implements: receiving an update for a database, wherein the update is part of a transaction; in response to receiving the update, generating a plurality of log records, wherein the plurality of log records comprises: a redo log record for a data page modified by the update; a log record for an undo page for the update; and a log record updating a transaction table page; performing a storage operation to persistently store the plurality of log records; subsequent to persistently storing the plurality of log records: determining that result of the log record for the undo page is not dependent on data of a prior version of the undo page and that result of the log record for the transaction table page is not dependent on data of a prior version of the transaction table page; maintaining the log record for the undo page and the log record for the transaction table page as log records to exclude the log record for the undo page and the log record for the transaction table page from a coalesce operation, wherein the coalesce operation applies updates indicated by a selected set of log records to a previous instance of a given page to generate a new instance of the given page; combining the redo log record for the data page modified by the update with one or more redo log records for the data page as part of the coalesce operation to create an instance of the data page; and performing a storage operation to persistently store the instance of the data page separate from the plurality of log records.
 20. The non-transitory, computer-readable storage medium of claim 19, wherein the log record for the undo page indicates the lack of dependency on the data of the prior version of the undo page, and wherein the log record for the transaction table page indicates the lack of dependency on the data of the prior version of the transaction table page.
 21. The non-transitory, computer-readable storage medium of claim 19, wherein a storage operation counter is maintained for the data page that indicates a number of redo records for the data page, and wherein said combining the redo log record for the data page modified by the update and said performing the storage operation to persistently store the instance of the data page are performed in response to determining that the storage operation counter indicates that that a storage operation threshold for the data page has been exceeded.
 22. The non-transitory, computer-readable storage medium of claim 21, wherein other respective storage operation counters are maintained for the undo page and the transaction table page, and wherein, in said maintaining the log record for the undo page and the log record for the transaction table page as log records, the program instructions when executed by the one or more computing devices further implement: resetting the respective storage operation counters for the undo page and the transaction table page.
 23. The non-transitory, computer-readable storage medium of claim 19, wherein one or more of the one or more computing devices executing the program instructions cause the one or more computing devices to implement said receiving the update and said generating the plurality of log records are implemented as part of a database service; wherein another one or more of the one or more computing devices executing the program instructions cause the other one or more computing devices to implement said performing the storage operation to persistently store the plurality of log records, said determining that result of the log record for the undo page is not dependent and that result of the log record for the transaction table page is not dependent, said maintaining the log record for the undo page and the log record for the transaction table page as log records, said combining the redo log records for the data page, and said performing the storage operation to persistently store the instance of the data page are implemented as part of a database service. 